Currently, a widely used method for detection of bleeding into feces is indexing hemoglobin derived from blood. Accordingly, a method of biochemical coloring reaction using hemoglobin's peroxidase-like activity, and a method of immunological detection using an antibody against hemoglobin (Songstar et al., Cancer. 45, pp. 1099-1102, 1980) are known. Even though the former biochemical method can detect blood from the digestive tract, there is always the possibility of a false-positive because a similar reaction may be caused by other substances with peroxidase-like activity or animal hemoglobin originating from diet. On the other hand, the latter immunological method will not be influenced by animal hemoglobin or substances with peroxidase-like activity originating from diet. The immunological method with those characteristics is widely used because testing can be done at any time without restricting meals.
Even though the immunological method of detecting fecal occult blood has been successful in terms of specificity and sensitivity, even now there are several problems raised about that method. The biggest problem is the hemoglobin's stability while it is in the digestive tract, and its stability at the time of preservation or transportation of the fecal samples. Feces contain several factors which will significantly affect the hemoglobin's immunological activity. In the case of bleeding in an upper part of the digestive tract, hemoglobin's detection sensitivity will be strongly influenced by digestion by digestive enzymes, the biological dissolution by microorganisms, or chemical transformation under acidic conditions. There are cases in which no hemoglobin is detected in spite of bleeding because the hemoglobin's immunological activity was weakened or lost due to a non-specific adhesion of various kinds of solid substances. Thus, bleeding detection techniques using hemoglobin as an indicator can detect bleeding in a lower part of the digestive tract. However, there is a stability problem when used as an indicator of bleeding in the upper part of the digestive tract. Furthermore, there is also a possibility of false negative detection due to unknown factors that cannot be explained simply by the above causes. The solutions to these problems are in great demand.
Another problem with the immunological method is to maintain specificity. The immunological method accomplishes a higher specificity, compared with the classical method using hemoglobin's chemical activity as an indicator. It also demands strict conditions of quality control for the reagent because its higher specificity strongly depends on the characteristics of antibodies.
As mentioned above, difficult problems still remain unsolved in the immunological method. They are: (1) an oversight of bleeding in the upper part of the digestive tract, (2) stability problems in preservation, neglect, and transportation of the samples; and (3) difficulty in reagent control. Therefore, there is a strong expectation of new technology which can easily accomplish superior specificity to the immunological method.
The other substances are tested as an indicator to solve the hemoglobin stability problem. For example, the trypsin inhibitor, .alpha.1-anti-trypsin (.alpha.1AT), has been used as an indicator for detecting bleeding into the feces. However, it was pointed out that .alpha.1AT can be detected in feces due to causes other than bleeding. It is not widely used because its stability in the digestive tract or the feces was proved to be less than that of hemoglobin. On the other hand, genes have been used as an indicator for analysis of organisms. For example, Kourilsky and others disclosed a method for detection of nucleic acid with an enzyme labeled probe (U.S. Pat. No. 4581333). Ranki disclosed a sandwich hybridization assay method (U.S. Pat. No. 4489839). Those technologies are widely used for detecting nucleic acid of genetic composition, and they are applied to a variety of the samples. Many reports on the detection of genes related to cancers, viruses, and germs are presented. There is a report on an experiment to detect cancerous genes in feces with an application of this technology (Science 256; 102-05,1992).